This invention relates, in general, to ion implantation systems and, more particularly, to an improved ion implanter end processing station.
The usual ion implantation facility consists of a source of ions, an accelerator and an end processing station. In the device there are two basic ion sources available for use, one source utilizes a 100 MHZ 200 watt oscillator to drive a plasma in a pyrex tube filled with a gas which contains the desired atomic species. Gas is fed into the plasma tube, accelerated and then formed into a beam before it is caused to impinge on the device to be implanted with the transported atomic species. The system is maintained under a vacuum by means of diffusion pumps for the source/accelerator section, the drift tube section and the target or end chamber. To maintain a vacuum in each area, one gate valve is placed in the drift tube down stream of the accelerator and one valve placed upstream of the target chamber. The target chamber is provided with a pump. The accelerated beam containing the ion species (which species represents the common dopant atom) is caused to impinge on the wafers to be implanted, that are contained in the end chamber.
In one embodiment the wafers are usually mounted on a carousel and arranged around the periphery thereof while the carousel is caused to rotate. As the carousel rotates, the beam is deflected and thus caused to scan across each wafer. After a predetermined time, which will dictate the amount of dopant atoms implanted into each wafer, the gate valve between the drift tube and the end chamber is closed to seal the end chamber from the remainder of the implantation system and the end chamber is exhausted to the atmosphere. The carousel is then removed and replaced with another carousel having wafers arranged around the periphery thereof.
It has long been recognized by the art that in order to increase production of implanted wafers that if a pair of carousels were placed in the chamber, a greater number of wafers can be implanted during the same pumped down and exhaust cycle. The prior art now provides a pair of back-to-back carousels, each containing about twenty 4" wafers, arranged on each carousel, and means to shift the position of each carousel into the path of the accelerated beam. With such an arrangement, it would take about 15 minutes to pump the end chamber down to the required vacuum, about 4 minutes to process each carousel and about 4 minutes to exhaust the chamber. Thus, about 40 wafers could be processed in about 27 minutes. It should be understood, however, that if the wafers were smaller, i.e., 2" wafers, a greater number of wafers could be processed but the system would still be limited to a 15 minute pump down time, an 8 minute implantation time and a 4 minute exhaust time and would still, thus, require a 27 minute cycle. The 15 minute pump down time is required because of the large volume required when two carousels are mounted back-to-back. In this arrangement, the wafers on a first carousel, for example the upper carousel, are first processed then both carousels are moved vertically so that the wafers in the upper carousel are out of the view of the incoming beam while the wafers on the lower carousel are now in the place previously occupied by the upper carousel. The wafers on the lower carousel are then processed. The resultant chamber necessary to accommodate these carousels thus dictates the need for the inordinately long pump down time and the accompanying long exhaust time.